Cardiac myosin was analyzed in dogs where a pressure overload was produced either by pulmonic or aortic banding, for varying lengths of time. When the banding was mild, such as in mild aortic stenosis (peak systolic pressure gradient being approximately 25 mm Hg) myosin elevated in enzymatic V max values. Myosin enzymatic function peaked 5 weeks post-operatively and returned to near normal values 16 weeks after surgery. When aortic banding was severe, affecting a peak systolic transaortic pressure gradient of 50 mm Hg, myosin decreased in activity and continued to decrease until the animals were in congestive heart failure. The left ventricular enzymatic V max values by 16 weeks post-operatively declined to those similar to the normal right ventricle. When excess pressure was placed on the right ventricle, by banding the pulmonary artery in dogs, alterations also occurred in myosin enzymatic activity. If banding was mild, increasing right ventricular peak systolic pressure 60% above normal, myosin increased in activity, peaked in enzymatic V max values by 5 weeks post-operatively and returned to near normal values of right ventricular myosin 16 weeks after surgery. If pulmonary banding was severe, increasing right ventricular peak systolic pressure 300% above normal, myosin decreased in enzymatic V max values. We are interested in assessing what molecular alterations occur in myosin to account for these changes in hydrolysis of ATP. This is important since the rapidity and degree of muscle shortening depend, at least in part, on the rate myosin hydrolyses ATP. In further studies we are interested in determining whether hypertrophy and thus alterations in myosin are reversible, and establishing an immunoabsorption assay to assess cardiac hypertrophy. In these comparative studies the biochemical aspects to be pursued relative to myosin are determination of: (a) Degree of calcium binding, (b) Light to heavy chain stoichiometry, (c) Phosphorylation, (d) Charge difference in light chains, (e) CNBr peptide mapping of heavy chains, and (f) Immunological comparative analyses.